This application claims priority to an application entitled xe2x80x9cElectro-Absorption Typed Optical Modulatorxe2x80x9d filed in the Korean Industrial Property Office on Mar. 23, 2000 and there duly assigned Ser. No. 2000-14821.
1. Field of the Invention
The present invention relates generally to an optical communication device, and in particular, to an electro-absorption semiconductor modulator.
2. Description of the Related Art
It is highly desirable to efficiently modulate optical signals in an optical communication system. As is known, electro-optical modulators absorb or transmit optical pulses sent from a laser source, on the basis of an electrical command, towards their surface. At their output duly coded digital signals can be obtained to be transmitted by optical fibers. There are two modulation techniques in optical telecommunications system, a direct optical modulation and an external optical modulation. The direct optical modulation modulates a signal by adjusting the current flowing into a laser diode, whereas the external optical modulation passes a light beam using a separate modulator.
Electro-optical modulators are typically fabricated on a III-V element substrate with a xe2x80x9cp-I-nxe2x80x9d structure. For example, InGaAs(P)/InGaAsP or InGa(Al)As/InAlAs is routinely used as a multi-quantum well (MQW) structure to absorb light in optical modulator. In the former case (where InGaAs(P)/InGaAsP is used for the MQW), the conduction band (CB) has a low energy band offset (of below 100 mV), which is shown in the energy band diagram of FIG. 1. In FIGS. 1 and 2, xe2x80x9caxe2x80x9d denotes a barrier layer, xe2x80x9cbxe2x80x9d denotes a quantum well layer, xe2x80x9cVxe2x80x9d denotes an energy band offset of the conduction band, xe2x80x9cExe2x80x9d denotes a first energy level of the electrons in the conduction band, xe2x80x9cLxe2x80x9d denotes a width of the barrier layer, xe2x80x9cCBxe2x80x9d denotes a conduction band, and xe2x80x9cFxe2x80x9d denotes a forward electric field. Accordingly, when an electric field is applied to such an optical modulator, a rectangular barrier layer 62 indicating the first energy level of electrons in the conduction band changes to a triangular barrier layer 64, as shown in FIG. 2. In this case, a bonding force among the electrons in the conduction band inside the quantum well is drastically reduced, resulting in a drastic decrease in an optional absorption force (or absorption threshold shifts) in the optical modulator.
The present invention relates to an improved electro-absorption semiconductor modulator in which a multi-quantum well (MQW) structure for absorbing a light beam is configured to increase an optical absorption capability according to the electric field therein, thus increasing the optical power difference between ON and OFF states.
Accordingly, there is provided an electro-absorption semiconductor modulator comprising a semiconductor substrate; a lower cladding layer deposited over the semiconductor substrate; a multi-quantum well (MQW) layer comprised of barrier layers and undoped well layers stacked in succession on the lower cladding layer, the barrier layers are doped with an impurity; an upper cladding layer deposited over the multi-quantum well layer; and, an ohmic contact layer deposited over the upper cladding layer.
Preferably, the impurity comprises an n-type dopant such as silicon.
Preferably, the impurity has a doping density of 1017/cm3.